The myth of an easy conquest in Mexico can be broken down into several parts. For this answer I will draw from previous posts, and I heartily recommend Restall's Seven Myths of the Spanish Conquest which is the best engaging introduction (and from which I will borrow extensively).

First, the rules of conquest mean there are no unbiased narrators. Cortes and Castillo's accounts are, in essence, a résumé of their exploits. Like all good résumés, there are elements of truth, but the single goal of each word is aggrandizing ones own exploits, and ignoring the contributions of everyone else. After conquistadores completed their battles, they were required to provide a review of their exploration, conquest and settlement. These probanza de mérito served to update the monarch on events in newly conquered land, as well as petition for rewards like offices, titles and pensions. Since the document required authors to lobby for their own gain, probazas naturally paint the author in the best, most courageous light, while ignoring the influential role of other conquistadores, native allies, and pure dumb luck. The greatest rewards went to the best shameless self-promoters, and the rules of the game rewarded those willing to stab their compatriots in the back (sometimes literally) in the hope of future gain.

Probanzas written by hopeful conquistadores looking for reward flooded back to Spain. The document developed its own genre, with its own accepted writing style, format, and rules of construction as thousands arrived at the court in Seville. Probanzas evolved into chronicles, like Bernal Diaz’s Conquest of Mexico, and provided justification not only for the actions of individual conquistadores, but also, in a larger sense, justified imperial expansion to bring civilization and Christianity to the New World. Uncritically examining these documents for a history of conquest is akin to writing a biography based on your embellished résumé. Sure, some elements of reality emerge, but when the whole point is to make yourself, your monarch, and your god look supreme, truth becomes a flexible concept.

Unlike the majority that were likely not even seen by royal eyes, Cortés’s cartas were published and translated into five languages, grew immensely in popularity, and were subject to royal ban as his cult of personality became a political threat to the crown. His status only increased with Gomara’s hagiography in 1552. Again, the crown tried to suppress that as well. Among modern English speakers, we inherit the cult of Cortés, and to a lesser extent Pizarro, through Prescott’s The History of the Conquest of Mexico (1843), and The History of the Conquest of Peru (1847). Prescott used the *cartas, probanzas, and previous works like Gomara’s to produce a story of conquest rooted in the mindset of imperial expansion common to mid-nineteenth century Americas.

Second, because the reward system favored shameless self-promotion, and we typically only read Spanish accounts, we conveniently forget about the central role of native allies. Put bluntly, the conquest of Mexico, and later in Peru, could be interpreted as totally indigenous civil wars, with some conquistadors randomly thrown in for chaos. The Mexica allied with Texcoco and Tlacopan to form the Triple Alliance in 1430, and over the next ninety years engaged in a series of conquests that expanded their area of influence in central and southern Mexico. Some city-states, like Tlaxcala, managed to maintain their independence but constantly lived in the shadow of an aggressive, expansive neighbor. Once conquered, subordinate city-states, and their elite ruling class, typically remained intact after incorporation into the empire. Local elites controlled the tribute, and were often integrated into the Triple Alliance Empire ruling class through marriage. Personal loyalty was directed at the city-state, not the greater empire, and frequent revolts required the rapid deployment of soldiers from the core cities to quell rebellions. Through the period of expansion the Mexica gradually rose to preeminence among the three original alliance members.

Cortés exploited the political situation and marched on Tenochtitlan with the Tlaxcala army of ~6,000 warriors (Gómara’s estimate). Everything went wrong when he decided to capture Moctezuma and hold him for ransom in his own capital. Pánfilo de Narváez landed on the coast to arrest Cortés for mutiny/treason, so Cortés left to fight Narváez, only to find that in Tenochtitlan a new leader, Cuitláhuac, was elected in place of the captive Moctezuma. When the Spanish murdered Moctezuma, and many Mexica elites, the fragile peace dissolved and the capital erupted in violence. 600 Spaniards died in the retreat on Noche Triste to the refuge of Tlaxcala. Far from providing Cortés free room and board, the Tlaxcala demand a share of the spoils once Tenochtitlan fell, as well as the city of Cholula, freedom from future taxes, and the right to build a citadel in Tenochtitlan. Cortés was in no place to refuse. His weakened soldiers couldn’t fight on, and charges of mutiny/treason awaited his return to Cuba even if he returned. With ingenuity born of absolute necessity, he aided the Tlaxcala in planning the attack against Tenochtitlan. Six months of Tlaxcala plotting and accruing allies followed. The final force brought to the siege of Tenochtitlan included the Tlaxcala, Texcoco, Huexotzinco, Atlixco, Chalca, Alcohua, and Tepanecs. After two and a half months of siege aided by a disastrous smallpox epidemic, a massive Native American army and the Cortés auxiliaries entered Tenochtitlan and destroyed the city. All told, <2,000 Spaniards and ~200,000 native allies fought in the two year campaign. If chaos is a ladder, Cortés real genius was positioning himself on the top rung when the capital finally fell.